1. Field of the Invention
The present invention relates to an antenna apparatus for a mobile terminal and in particular, to an antenna apparatus for a mobile terminal where the antenna apparatus includes a hula hoop antenna and an extendable rod antenna for operating the antenna apparatus in an extended and in a retracted state.
2. Description of the Related Art
An antenna apparatus for a mobile terminal generally includes a fixed helical antenna and a retractable rod antenna. The helical antenna operates in a retracted state and the rod antenna operates in an extended state.
FIGS. 1 and 2 illustrate a prior art antenna apparatus having a rod antenna in the extended state and in the retracted state, respectively. A detailed discussion of the structure and operation of the antenna apparatus shown by FIGS. 1 and 2 can be found in Korean Patent Registration No. 107414/1996.
The antenna apparatus for a mobile terminal, as illustrated by FIGS. 1 and 2, includes a helical antenna 130 mounted on an upper portion of a housing 301, and a rod antenna 120 fixed to the housing 301 by an antenna cap 106. The hollow antenna cap 106 has a protrusion 107 formed at an upper, inner wall through which the rod antenna 120 is inserted. Under the antenna cap 106, a conductive female screw 111 is fixed to the upper end of the housing 301. A cylindrical male screw 109 having a through hole is screwed to the female screw 111. A head of the cylindrical male screw 109 is attached to a lower end of a helical winding 108 inserted into an opening of the antenna cap 106. The antenna cap 106 is fixed to the housing 301 such that a lower end of the antenna cap 106 is fixed to the head of the cylindrical male screw 109.
The rod antenna 120 is composed of a polyacetal rod 104, an antenna core line 105 inserted into the polyacetal rod 104, an isolation element 103 with a fixing groove 102 formed at an upper, outer circumference, and a pull 101 formed at an upper end of the isolation element 103. The rod antenna 120 is inserted into the antenna cap 106, passing along a central axis of the helical antenna 130 and the through hole of the cylindrical male screw 109. A lower end of the polyacetal rod 104 is fixed to a stopper 110.
In the retracted state of the rod antenna 120, the protrusion 107 formed at the upper portion of the antenna cap 106 is inserted into the fixing groove 102 of the isolation element 103 so that the rod antenna 120 may not extend out of the antenna cap 106 by itself. In the extended state of the rod antenna 120, the stopper 110 fixed to the lower end of the polyacetal rod 104 is stopped by a plate spring 112 mounted on the through hole of the cylindrical male screw 109. The female screw 111 is connected to a printed circuit board (PCB) 205 via a feeding connector 201.
With continued reference to FIGS. 1 and 2, the antenna core line 105 extends from the stopper 110 to the lower end of the isolation element 103. The polyacetal rod 104 has a good restoring force and serves as a protection rod for the antenna core line 105. The antenna core line 105 may be made of a silver-plated cooper wire or piano wire, or a super-elastic nickel-titanium wire (i.e., shape-memory alloy wire) having a good restoring force. An electric length of the antenna core line 105 measures between .lambda./4 and .lambda./2 (i.e., approximately 87-174 mm at 860 MHz), taking into consideration the vertical length of the housing 301. In practice, a physical length of the antenna core line 105 can be reduced to 132 mm by virtue of a dielectric constant indicative of the polyacetal rod 104. When the vertical length of the housing 301 is very short, a telescoping antenna may be used for the rod antenna 120.
The helical winding 108 of the helical antenna 130 is made of a silver-plated piano wire having a diameter of approximately 5.6 mm. An electric length of the helical winding 108 is related to the length of the antenna core line 105 of the rod antenna 120. A physical length of the helical antenna 130 is relatively much shorter than that of the rod antenna 120.
The antenna apparatus is positioned at one side of the mobile terminal, i.e., there is a positional asymmetry in the placement of the antenna apparatus with respect to the housing of the mobile terminal. In the extended state of the rod antenna 120, the positional asymmetry of the antenna apparatus, specifically, the positional asymmetry of the helical antenna 130 is non-problematic, since in the extended state, the overall length of the antenna apparatus is increased. Therefore, the distribution of radiation and the quality of communication is typically not affected.
However, in the retracted state of the rod antenna 120, the overall length of the antenna apparatus is reduced and only the helical antenna 130 radiates a radio signal. As a result, due to the positional asymmetry of the helical antenna 130, a radiation pattern indicative of the radiated radio signals is distorted, i.e., the radiation pattern is asymmetrical, thereby reducing the distribution of radiation in one or more directions. Hence, a receiving sensitivity may depend on the position of the mobile terminal.
Further, an increase in the operating frequency requires an extension in the size of the mobile terminal with respect to the wavelength which accelerates the distortion of the radiation pattern, thereby presenting a difficulty in designing a compact mobile terminal. To counteract this phenomenon, the length of the antenna apparatus can be increased. However, an increase in the length of the antenna apparatus presents a difficulty in designing a compact mobile terminal.
Therefore, a needs exists to provide a non-directional antenna apparatus for a mobile terminal which has a stable receiving sensitivity in the elevation and azimuth planes without compromising the compactness of the mobile terminal.
Further, a need exists to provide an antenna apparatus for a mobile terminal which has a symmetric radiation pattern in the retracted and extended states.